Carbon fibers are lightweight but are excellent in strength and elastic modulus, and thus are combined with various matrix resins to form composite materials, which are used in various fields including aircraft members, spacecraft members, automobile members, ship members, constructional materials, and sporting goods. In a composite material formed by using carbon fibers, excellent interfacial adhesion between the carbon fibers and a matrix resin is important in order to impart excellent characteristics of the carbon fiber.
In order to improve the interfacial adhesion between the carbon fibers and the matrix resin, an oxidation process such as gas phase oxidation and liquid phase oxidation is usually applied to carbon fibers to introduce oxygen-containing functional groups to the surface of the carbon fiber. For example, a method for improving interlaminar shear strength that is an index of the interfacial adhesion by carrying out electrolytic treatment to carbon fibers is disclosed (refer to Patent Literature 1). However, as required characteristics of composite materials have become higher in recent years, the interfacial adhesion achieved by such oxidation alone has become insufficient.
On the other hand, the carbon fibers are brittle and poor in bindability and abrasion resistance and thus easily generate fluffs or broken threads in a high-order processing step. To solve these disadvantages, a method for coating carbon fibers with a sizing agent is disclosed (refer to Patent Literatures 2 and 3).
For example, as a sizing agent, an aliphatic compound having a plurality of epoxy groups is disclosed (refer to Patent Literatures 4, 5, and 6). A method for coating carbon fibers with an epoxy adduct of polyalkylene glycol as a sizing agent is disclosed (refer to Patent Literatures 7, 8, and 9).
A method for coating carbon fibers with a diglycidyl ether of bisphenol A as an aromatic sizing agent is disclosed (refer to Patent Literatures 2 and 3). A method for coating carbon fibers with a polyalkylene oxide adduct of bisphenol A as a sizing agent is disclosed (refer to Patent Literatures 10 and 11). A method for coating carbon fibers with a sizing agent that is obtained by adding an epoxy group to a polyalkylene oxide adduct of bisphenol A is disclosed (refer to Patent Literatures 12 and 13).
Although the sizing agents described above can impart adhesion and bindability to the carbon fibers, the sizing agent including one epoxy compound is insufficient, and thus, in recent years, a technique of using two or more epoxy compounds in combination has been developed depending on required functions.
For example, a sizing agent including two or more epoxy compounds having defined surface energies is disclosed (refer to Patent Literatures 14 to 17). Patent Literature 14 discloses a combination of an aliphatic epoxy compound and an aromatic epoxy compound. Patent Literature 14 describes that the sizing agent existing in the outer layer in a large amount has an effect of shielding the sizing agent component existing in the inner layer in a large amount from air, and this prevents the epoxy group from opening its ring by moisture in air. Patent Literature 14 also describes that the sizing agent preferably contains the aliphatic epoxy compound and the aromatic epoxy compound in a ratio of 10/90 to 40/60, and the aromatic epoxy compound is preferably contained in a larger amount.
Patent Literatures 16 and 17 disclose a sizing agent formed by using two or more epoxy compounds having different surface energies. Patent Literatures 16 and 17 aim to improve adhesion to a matrix resin and thus do not limit the combination of two or more epoxy compounds to the combination of an aromatic epoxy compound and an aliphatic epoxy compound, and describe no typical example of the aliphatic epoxy compound selected from the viewpoint of adhesion.
A sizing agent containing a bisphenol A epoxy compound and an aliphatic polyepoxy resin in a mass ratio of 50/50 to 90/10 is disclosed (refer to Patent Literature 18). However, the sizing agent disclosed in Patent Literature 18 also contains the bisphenol A epoxy compound as an aromatic epoxy compound in a large amount.
As a sizing agent specifying the combination of an aromatic epoxy compound and an aliphatic epoxy compound, a combination of a multifunctional aliphatic compound on the surfaces of carbon fiber bundles and an epoxy resin, a condensate of an alkylene oxide adduct with an unsaturated dibasic acid, and an alkylene oxide adduct of phenols on the surface of the multifunctional aliphatic compound is disclosed (refer to Patent Literature 19).
As a combination of two or more epoxy compounds, a combination of an aliphatic epoxy compound and a bisphenol A epoxy compound as an aromatic epoxy compound is disclosed. The aliphatic epoxy compound is a cyclic aliphatic epoxy compound and/or a long chain aliphatic epoxy compound (refer to Patent Literature 20).
A combination of epoxy compounds having different properties is also disclosed. A combination of two epoxy compounds that are liquid and solid at 25° C. is disclosed (refer to Patent Literature 21). Furthermore, a combination of epoxy resins having different molecular weights and a combination of a monofunctional aliphatic epoxy compound and an epoxy resin are disclosed (see Patent Literatures 22 and 23).
However, the sizing agents (for example, Patent Literatures 20 to 23) containing two or more components practically provide insufficient improvement of physical properties of the molding material containing a sizing agent-coated carbon fibers and a matrix resin such as a thermoplastic resin. The following two requirements are needed to be satisfied in order to satisfy high adhesion between carbon fibers and the thermoplastic resin. However, it is supposed that sizing agents made of conventional combinations of any epoxy resins have failed to satisfy these requirements. In the two requirements, the first is that an epoxy component having high adhesion exists in the inner side (a carbon fiber side) of a sizing layer, and the carbon fibers and the epoxy compound interact strongly and the second is that the surface layer (a side of a thermoplastic resin as a matrix resin) of the sizing agent requires a chemical composition capable of strongly interacting with the epoxy compound having high adhesion to the carbon fibers existing in the inner layer and a thermoplastic resin in the outer layer.
For example, Patent Literature 14 discloses a sizing agent having a gradient structure for improving the adhesion between carbon fibers and a sizing agent. However, Patent Literature 14 and any other literatures (for example, Patent Literatures 15 to 18) have no idea that, in a molding material containing a sizing agent-coated carbon fibers and a thermoplastic resin, the interfacial adhesion between the carbon fibers and the thermoplastic resin is improved by arranging a component having high adhesion in the inner layer of the sizing layer and arranging a component having high interaction with the thermoplastic resin in a surface layer of the sizing layer.
Patent Literature 19 discloses a sizing agent in which a multifunctional aliphatic compound exists in an inner layer of the sizing agent and an aromatic epoxy resin and an aromatic reaction product each having low reactivity exist in an outer layer. However, the aliphatic compound and the aromatic compound are separated and thus high adhesion is difficult to achieve.
As described above, in conventional techniques, when a thermoplastic resin is particularly used as a matrix resin, the interfacial adhesion between the thermoplastic resin and the carbon fibers is poor and a further interfacial adhesion improvement technique is required.